The present invention generally relates to signal limiters used to prevent clipping or saturation, or to control the characteristics of a signal around some maximum amplitude, and particularly relates to multichannel limiters.
Signal limiting represents a common safeguarding function that is employed in a wide variety of applications, in the analog, digital and acoustic domains. Signal limiting in its simplest form comprises limiting a given signal's amplitude at some maximum permissible value. Hard limiting, for example, imposes a hard limit on the peak signal amplitude permitted, with the hard limit typically set at the value needed to prevent over-driving one or more circuits in the signal path into saturation. Hard limiting may occur inherently, such as when the value of a signal exceeds the full-scale measurement range available for measuring the signal.
In any case, hard limiting typically “clips” a given waveform at the limit, which changes the waveform shape, forfeits signal information, and introduces signal non-linearity. Soft limiting represents a more sophisticated approach to limiting. Soft limiting preserves the shape of the signal, while still limiting its peak amplitude. In other words, soft limiting preserves the linearity of the signal by fractionally scaling it—i.e., attenuating the signal by some gain factor less than unity—rather than simply capping its peak values. Not surprisingly, then, soft limiting represents the preferred approach in applications where the non-linearity that would be introduced by hard limiting is objectionable. Audio signal limiting is one such application.
For example, soft limiting can be used on a stereo signal to prevent either channel signal from exceeding some defined magnitude limit. Soft limiting in audio applications prevents undesirable non-linearity in the audio signal, which would result in audible distortions. The magnitude limits may be defined by one or more digital or analog circuit elements in the audio signal path, or by the loudspeakers being used to generate the audible output, for example.
Regardless of the underlying reason for imposing magnitude limits, the application of limiting control to a multichannel signal can be problematic. For example, only a subset of the channel signals comprising a multichannel audio signal may require limiting at any given time, or different ones of the channel signals may require differing amounts of soft limiting to prevent clipping. In either case, the conventional approach to limiting control applies to each channel whatever soft scaling value is needed to prevent that channel signal's amplitude from exceeding the limits defined for that channel.
In other words, different limiting values potentially are applied to different ones of the channel signals comprising the multichannel signal, meaning that the relative amplitude relationships that existed among the channel signals before limiting is altered by the limiting operation. With multichannel audio signals, altering the relative amplitudes among the channel signals causes unintended and oftentimes undesirable effects. For example, the stereo or multichannel “image” being produced by the loudspeakers may noticeably shift around as a function of limiting control. Other types of deleterious effects may arise in other multichannel signal limiting applications.